Position tracking and imaging system with error detection for use in medical applications

ABSTRACT

A system for monitoring the position of a medical instrument with respect to a patient&#39;s body and for displaying at least one of a plurality of prerecorded images of said body responsive to the position of said medical instrument. In one embodiment the system includes a reference unit secured from movement with respect to the patient&#39;s body such that said reference unit is substantially immobile with respect to a target operation site. The system also includes a remote unit for attachment to the medical instrument. A field generator may be associated with one of the units for generating a position characteristic field in an area including the target operation site. One or more field sensors may be associated with either of the units responsive to the presence of the position characteristic field for producing one or more sensor output signals representative of said sensed field. A position detector in communication with the sensor output signal produces position data representative of the position of the remote unit with respect to the reference unit. An output display in communication with the position detector displays at least one of the prerecorded images responsive to the position data.

This is Divisional of U.S. Ser. No. 08/527,517 filed on Sep. 13, 1995,now pending, which is a Continuation-In-Part of Ser. No. 08/306,818filed on Sep. 15, 1994, now pending.

BACKGROUND OF THE INVENTION

The invention relates to computer assisted medical surgery and inparticular relates to systems for displaying prerecorded visual imagesduring surgical operations.

Presently available medical imaging techniques-such as CAT (ComputerizedAxial Tomography), MRI (Magnetic Resonance Imaging), and PET (PositionEmission Tomography), are known to be helpful not only for diagnosticpurposes, but also for providing assistance during surgery. Prerecordedimages may be displayed during surgical operations to provide thesurgeon with illustrative reference mappings of pertinent portions of apatient's body.

Tracking systems for monitoring the position of a medical instrumenthave also been developed for use with image display systems. Generally,as the surgeon moves the medical instrument with respect to thepatient's body, associated prerecorded images are displayed responsiveto the movement of the instrument. Such tracking systems typicallyinvolve either the use of a passive articulated arm attached to themedical instrument, optical detection or ultrasonic detection.

Tracking systems using a passive articulated mechanical arm attached toa medical instrument are disclosed in U.S. Pat. Nos. 5,186,174 and5,230,623. Generally, as the surgeon moves the surgical instrument withrespect to the patient's body, micro recorders at the joints of thearticulated arm record the respective amounts of movement of each armmember. The outputs of the micro recorders are processed and theposition of the medical instrument with respect to the base of thearticulated arm is thereby monitored. One or more prerecorded images arethen displayed responsive to the movement of the surgical instrument.Such articulated arm tracking systems, however, require that theinstrument be attached to a cumbersome mechanical arm. Also, althoughfree movement of the tip of the arm in three dimensional space may betheoretically possible, the surgeon might experience difficultypositioning the instrument at certain locations and in desiredorientations within the body.

Tracking systems using optical detection (video cameras and/or CCDs(Charge Coupled Devices)) have been proposed for monitoring the positionof a medical instrument with respect to a reference unit as mentioned inU.S. Pat. No. 5,230,623. Such systems, however, require that thereference unit and the instrument both be within the view of the camera.This not only limits the movement of the surgical staff, but alsorequires that at least a portion of the medical instrument remainoutside the patient's body.

Tracking systems using ultrasonic detection are generally disclosed inU.S. Pat. No. 5,230,623. Such systems, however, are disclosed to be usedin a fashion similar to optical detection, i.e., triangulation oftransmitted signals. The transmitted signals are sent from one or moresenders to associated receiver(s), and the distances travelled by thesignals are determined from either timing or amplitude changes. Again,the transmission path must remain unobstructed.

A further shortcoming common to each of the above tracking systems isthat the patient must not move during the operation. Although thepatient is likely to be generally anesthetized, the patient's body maybe inadvertently moved by the surgical staff, or the surgeon may want tomove the body for better positioning. If the body is moved after thetracking system has been initialized, then the tracking will bemisaligned.

There is a need therefore for a system for monitoring the position of amedical instrument with respect to a patient's body that avoids theseand other shortcomings of present devices.

SUMMARY OF THE INVENTION

The invention relates to a system for monitoring the position of amedical instrument with respect to a patient's body and for displayingat least one of a plurality of prerecorded images of the body responsiveto the position of the medical instrument. The system includes areference unit, a remote unit, a position characteristic fieldgenerator, a field sensor, a position detection unit and an outputdisplay.

In one embodiment, the reference unit is secured from movement withrespect to at least a portion of the patient's body such that thereference unit is substantially immobile with respect to a targetoperation site. The remote unit is attached to the medical instrument.The field generator is associated with one of the reference or remoteunits and generates a position characteristic field, such as amultiplexed magnetic field, in an area including the target operationsite. The field sensor is associated with the other of the reference orremote units and is responsive to the presence of the field forproducing a sensor output signal representative of the sensed field.

The position detection unit is in communication with the sensor outputsignal and produces position data representative of the position of theremote unit with respect to the reference unit. The output display unitis in communication with the position detection unit for displaying atleast one of the prerecorded images responsive to the position data.

The system further may include a registration unit in communication witha storage unit and the position data. The storage unit stores theplurality of prerecorded images of the body. Each prerecorded image isrepresentative of a planar region within the body such that theplurality of planar regions represented by the prerecorded images definea first coordinate system. The registration unit correlates the positiondata of a second coordinate system (as defined by the position detectionunit) with the plurality of prerecorded images of the first coordinatesystem, and identifies a desired prerecorded image associated with theposition of the remote unit with respect to the patient's body.

The invention also relates to a reference unit that is attachable to apatient's head, and a medical instrument, such as an aspirating device,that is adapted to removably receive a position detection unit.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the invention may be furtherunderstood with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic view of a system of an embodiment of theinvention;

FIG. 2 is a front view of the headset unit shown in FIG. 1;

FIG. 3 is a side view of the headset unit shown in FIG. 1 taken alongline 3--3 of FIG. 2;

FIG. 4 is a rear view of a portion of the headset shown in FIG. 1 takenalong line 4--4 of FIG. 3;

FIG. 5 is an exploded side view of the surgical instrument and remotesensor shown in FIG. 1;

FIG. 6 is an end view of the assembled surgical instrument and sensorshown in FIG. 1 taken along line 6--6 of FIG. 5;

FIG. 7 is a side view of another embodiment of a surgical instrument andsensor unit of the invention in accordance with an alternativeembodiment of the invention;

FIG. 8 is a side view of the surgical instrument shown in FIG. 7;

FIG. 9 is an end view of the surgical instrument shown in FIG. 7;

FIG. 10 is an elevational view of the surgical instrument shown in FIG.7;

FIG. 11 is a plan view of a remote sensor unit that is adapted to beused with the surgical instrument shown in FIGS. 7-10;

FIG. 12 is a side view of another surgical instrument together with theremovable remote sensor unit shown in FIGS. 7 and 11;

FIG. 13 is a diagrammatic illustration of the system employed toprerecord CT images for use with the system of the invention;

FIG. 14 is diagrammatic illustration of a manual registration process ofthe invention;

FIG. 15 is an elevational view of the components of a fiducial markersystem in accordance with an embodiment of the invention;

FIG. 16 is a plan view of the components of the system of FIG. 15 takenalong line 16--16 thereof;

FIG. 17 is a flowchart of the process of using the fiducial markersystem of FIG. 15;

FIG. 18 is a side view of a headset unit in accordance with anotherembodiment of the invention;

FIG. 19 is an end view of the headset unit shown in FIG. 18 taken alongline 19--19 thereof;

FIG. 20 is a plan view of a transmitter that is adapted to be used withthe headset unit shown in FIG. 18;

FIG. 21 is a partial view of a portion of the headset shown in FIG. 16taken along line 21--21 thereof;

FIG. 22 is a flow chart of an automatic registration process of theinvention;

FIG. 23 is a diagrammatic view of the position detection components inaccordance with a system of the invention;

FIGS. 24 and 25 are diagrammatic views of the principles of an errordetection calculation process in accordance with an embodiment of theinvention;

FIGS. 26 and 27 are diagrammatic views of the errors detected by theprocess of FIGS. 24 an 25;

FIG. 28 is a diagrammatic view of another embodiment of the invention;and

FIGS. 29-32 are diagrammatic views of further embodiments of systems ofthe invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

As shown in FIG. 1, a system 10 of the invention includes a headset 12mounted on a patient 14, a medical instrument 16, a control system 18and a display 20. The control system 18 includes a position detectionunit 22, a registration unit 24, and an image storage unit 26.

The image storage unit 26 stores sets of prerecorded images such as CAT,MRI or PET scan images. Each set of images may be taken along, forexample, coronal, sagittal or axial directions. As shown in FIG. 1, thedisplay 20 shows three images, a coronal image 21a, a sagittal image21b, and an axial image 21c. Text information may also be displayed asshown at 21d in FIG. 1.

As further shown in FIGS. 2-4, the headset 12 includes two ear mounts 28on side members 30, and a nose bridge mount 32 on a center member 34.The headset 12 should be made of a resilient plastic such that it may besnugly attached to a patient's head, and may be provided in a variety ofsizes. A primary objective of the headset is to provide a reference unitthat may be easily attached to and removed from a patient's head whereinthe headset may be repeatedly reattached in exactly the same place witha high degree of accuracy. In other embodiments, the side members 30 ofthe headset 12 may be rotationally attached to one another and the earmounts 28 may be biased toward one another. Further, the center member34 may be rotatable with respect to the side members 30 and biasedtoward the ear mounts 28 as well.

The headset 12 shown in FIGS. 1-4 also includes a reference unit 36connected to the position detection unit 22 via communication lines 38.The reference unit 36 may be releasably attached to the headset 12 byconventional clamp or fastening means. In one embodiment the referenceunit 36 may include a position characteristic field generator capable ofgenerating a multidirectional field in three dimensions and may involvethe use of either electromagnetic or ultrasonic waves. The positioncharacteristic field differs from the transmit/receive triangulationsystem, in part, because it does not rely on the comparison of onetransmitted signal with another as does triangulation. This permits thepath between the field generator and the remote sensor to be obstructedby materials that do not significantly alter the generated field. Forexample, the position of the medical instrument could be identified evenwhen the instrument is within the patient's body when the generatedfield is a magnetic field. Additionally, the reference unit may alsoinclude a reference sensor 37 to provide verification of proper systemoperation.

In the present embodiment the field generator includes threeorthogonally disposed magnetic dipoles (e.g., current loops orelectromagnets), and the orthogonally disposed magnetic fields generatedby each of the three dipoles are mutually distinguishable from oneanother (e.g., via either phase, frequency, or time divisionmultiplexing). The near-field characteristics of the multiplexedmagnetic fields may be relied upon for position detection, for exampleas generally described in U.S. Pat. No. 4,054,881. In alternateembodiments the field generator may be located somewhere other than onthe headset and the headset may include two field sensors 36,37. Whenthe distance between the sensors 36,37 is known, the second sensor maybe used to act as a backup or reference check for monitoring the properoperation of the system. If the sensed fields are inconsistent then anerror signal is displayed and/or sounded.

In other embodiments the headset 12 may be employed in systems based onthe triangulation of signals where the reference unit 36 includes one ormore signal transmitters and/or one or more signal receivers. In such atriangulation system, position detection is achieved by comparingcertain characteristics of one transmitted signal with those of a secondtransmitted signal to determine the relative distances travelled. Thetransmitted signals may be electromagnetic (e.g., radio, laser light orlight emitting diodes) or may be ultrasonic. The position of thepatient's head with respect to the surgical instrument may thereby bemonitored.

As shown in FIGS. 5 and 6 the medical instrument 16 may be an aspiratingdevice adapted to removably receive a remote sensor 40 for detecting,for example, the field generated by the position characteristic fieldgenerator. The sensor 40 may be held inside the instrument 16 by forcefit sizing or through the use of a resilient snap member in the wallopening 42. Since an aspirating device is commonly used in most surgicaloperations, incorporating the remote sensor into the aspirating deviceprovides the surgeon with a convenient position detection device thatdoes not clutter the operation site with unnecessary items. Theinstrument 16 may further include a second backup field sensor 41 forsystem error detection as discussed above with reference to the sensor37.

The remote sensors 40,41 are removable from the aspirating device andmay be interchangeably inserted into any of a variety of speciallyadapted surgical instruments. In the illustrated embodiment, the remotesensors 40,41 are received through an opening 42 in the proximal end ofthe instrument 16, and are connected to the position detection unit 22via communication lines 44. The sensors 40,41 may also each includethree orthogonally disposed dipole sensing elements for detecting thepresence of the field generated by the field generator. For example, inone embodiment, the field generator and the sensors each include threeorthogonally disposed electrical wire loops. The generator produces analternating current through one generator loop at a time thus generatinga time division multiplexed alternating electromagnetic field. Thesensor loop signals are each processed in synchronous timing with thegenerator loops to produce outputs responsive to each respectivealternating electromagnetic field.

The distal end of the instrument 16 includes a rigid aspirating tube 46having a flared tip 48. The position of the tip 48 with respect to thecenter of the remote sensor 40 is a known constant and may be easilyseen by the surgeon during surgery. The aspirating tube 46 is in fluidcommunication with an aspirating catheter 50 through the proximal end ofthe instrument 16 via internal channel 52 and a connector element 54.The aspirating catheter 50 (shown in FIG. 1) is connected to a vacuumaspirating unit (not shown).

In operation, the position detection unit monitors the position of themedical instrument 16 with respect to the reference unit 36. Theregistration unit 24 correlates the changes in position of theinstrument 16 with the spacial orientation of the stored images. As thesurgeon moves the medical instrument 16, images appear on the display 20responsive to the position of the medical instrument 16. This permitsthe surgeon to always have available the coronal, sagittal, and axialviews associated with the precise location of the tip 48 of theinstrument 16 regardless of whether the tip 48 is inside of the patient14. Moreover, since the field generator is attached to the patient'shead, the patient is free to be moved without loss of the trackingcapabilities. The display 20 may further identify the location of thetip 48 on each of the displayed images as shown at 56 in FIG. 1. Inother embodiments the orientation of the aspirating tube 46 may also beidentified on the displayed images. In further embodiments, a threedimensional composite image may be displayed based on the prerecordedimages.

As shown in FIGS. 7-11 another embodiment of a removable remote sensorunit 58 may be used with an aspirating device 60. The sensor unit 58,including two sensors 62,64 may be removably attached to the device 60by first engaging recesses 66 on the unit 58 with fingers 68 on thedevice 60. A tounge 70 on the unit 58 is then received between hingeposts 72 on the device 60, and finally secured in place by rotating thelock 74 from an open position as shown in FIG. 8 to a closed position asshown in FIG. 7. The lock 74 includes a recessed area at 76 adapted tofrictionally engage the tounge 70 on the sensor unit 58.

The sensor unit 58 further includes the ability to identify which of aplurality of medical instruments is attached to the sensor unit 58 atany time. Specifically, the unit 58 includes a plurality of Hall effecttransistors 78, and the medical instrument 60 includes one or more tinypermanent magnets 80. By the number and/or positioning of the magnets80, the transistors 78 identify which of the medical instruments isattached to the sensor unit 58.

For example, if all of the transistors 78 sense the presence of a magnet80 then the instrument 60 shown in FIGS. 7-11 is known to be attached tothe sensor unit 58 since the instrument 60 includes three magnets. Ifonly two magnets 82 are sensed then the medical instrument attached tothe sensor unit 58 is a different instrument 84 as shown in FIG. 12. Ifno magnets are sensed then it is known that the sensor unit 58 is notattached to any medical instrument. Knowing the identity of the attachedmedical instrument permits the system to automatically adjust theposition detection unit to compensate for the differences in instrumenttip position with respect to the position of the sensors 62,64 for avariety of medical instruments. The removably engageable feature of thesensor unit not only provides versatility, but also facilitates the useof sterilized medical instruments.

As illustrated in FIGS. 13 and 14 the registration process involves twofundamental steps: 1) recording the scan images of a predeterminedorientation and 2) mapping the spacial orientation of the positiondetection system onto the recorded images. For example, the orientationsof the prerecorded images may be in the sagittal (i-j plane), coronal(k-j plane) and/or axial (k-i plane) as shown in FIG. 13. The images maybe digitally stored and the distance between each scanned image isrecorded, as are the relative orientations of each set of images. Asthose skilled in the art will appreciate, in alternative embodimentscertain of the images may be created from other images without the needto prerecord each of the sagittal, coronal and axial views. For example,by multiplanar reformatting the sagittal and coronal images may becreated from the axial images.

In one embodiment, fiducial markers 90 are placed on the patient's head14 prior to scanning with the scanner 92. The markers then appear oncertain of the scanned images, and may be located by the positiondetection system as shown in FIG. 14. Specifically, when each marker 90is sequentially located, for example with the tip 48 of a medicalinstrument 16, the user locates the same marker on the prerecordedimages by using, for example a computer mouse. The user then controlsthe entering of the registration data through either a computer keyboard94, a mouse, or a foot switch. In alternative embodiments theregistration unit may scan each prerecorded digital image beginning fromone corner until it locates the identified marker.

In further embodiments involving the use of fiducial markers that areplaced on the patient's body (e.g., face) prior to recording the scanimages, fiducial markers 90' may be adhered to intermediate adhesivestrips 91 which are directly adhered to the patient's skin 93 as shownin FIGS. 15 and 16.

The fiducial markers 90' include a radiopaque element 95 and the strips91 include a small puncture hole or other marker 97. With reference toFIG. 17, the process of using the fiducial markers 90' begins (step1700) by first placing the strips 91 on the patient's skin (step 1710).The fiducial markers 90' are then placed on the strips 91 such that theradiopaque elements 95 align with the markers 97 on the strips 91 (step1704). The scan images are then recorded (step 1706), and the fiducialmarkers 90' may then be removed from the patient (step 1708). Duringmanual registration the surgeon or technician may locate the markers 97with the tip of a pointer (step 1710) and thereby record the positionsof the fiducial marker radiopaque elements 95 with respect to thetransmitter. The use of the intermediate strips 91 not only providesincreased comfort to the patient after the image scanning and prior tosurgery, but also facilitates accurate registration. Since theradiopaque elements 95 were centered directly on top of the markers 93,the accuracy of registration is enhanced because the user may now locatethe smaller sized markers 93 instead of more indefinitely locating aportion of the larger sized radiopaque elements 95 with the pointer tip.

Once each of the markers has been located using the position detectionunit, the registration unit generates a mapping function to translatethe position detection data (in x-y-z coordinates) to the stored imageorientation data (in i-j-k coordinates). In particular, the mappingequation is determined by using Powell's method as follows.

The images points are each processed as a matrix of the form ##EQU1##and the collected sensor points are each processed as a matrix of theform ##EQU2##

A computer processor then iteratively calculates the optimal values forthe transformation matrices ##EQU3## to solve the following equation:##EQU4## such that (i_(c) -i_(i))² +(j_(c) -J_(i))² +(k_(c) -k_(i))² isa minimum for the summation of all of the collected image points. Theoptimization method employs distance minimization, and at least threeimage points are required for this method.

The optimal values for the transformation matrices comprise thetransformation equation and may now be used to translate the position ofthe medical instrument with respect to the transmitter in the x-y-zcoordinate system, to the appropriate orientation of the prerecordedimages in the i-j-k coordinate system.

A further embodiment of the headset of the invention may be employed inan automatic registration process. For example, as shown in FIGS. 18 and19 another embodiment of a headset 100 of the invention includes two earmounts 28, side members 30, and a nose bridge mount 32 on center member34 as discussed above with reference to FIGS. 2-4. The headset 100further includes a center plate 102 on the center member 34. The centerplate 102 is adapted to receive a transmitter 104 as shown in phantom inFIG. 19 and shown from the underside of the plate 102 in FIG. 21. Thetransmitter 104 includes two posts 106 and a key 108 that is free torotate about a pin 110.

To install the transmitter 104 on the center plate 102, the key ispassed through a longitudinal opening 112 in the plate 102, and theposts 106 are each received by post openings 114. One of the postopenings 114 is preferably formed as a slot to provide a snug fit forthe transmitter yet still accommodate variations between headsets due tomanufacturing tolerances. The key 108 may then be rotated to lock thetransmitter onto the outer facing surface of the plate 102. Thetransmitter 104 may then be removed from and reattached to identicalheadsets in the same location and orientation with a high degree ofaccuracy.

The headset 100 further includes very small (e.g., about 2 mm dia.)metal fiducial balls 116 secured within the center plate 102 as shown inFIG. 18. The automatic registration process locates the balls 116 on theprerecorded scan images, and knowing the spacial relationship betweenthe balls 116 and the transmitter 104, automatically generates themapping function to translate from the transmitter coordinate system tothe image coordinate system.

Specifically and with reference to FIG. 22, the automatic registrationprocess begins (step 2200) by loading the prerecorded images (step 2202)and then creating a three dimensional data set (step 2204). Pixelshaving an intensity within a certain range are then identified (step2206), and groups of adjacent pixels are located (step 2208) andclassified together as a single group. The volume of each group iscalculated (step 2210) and groups not within a predefined range ofvolumes are rejected (step 2212). Groups not having at least one pixelwith an intensity level of at least a certain amount are rejected (step2214). If the number of groups remaining is less than the number offiducial balls 116 (step 2216), e.g., 7, then the program ends havingfailed to provide automatic registration (steps 2218 and 2220).

The center of each group is then located and the distances between eachgroup's center and the other centers are calculated and recorded in amatrix of at least 7 by 7 (step 2222). The known distances between thefiducial balls comprise a predefined 7 by 7 matrix. The program thencompares each of the known distances with the various predefineddistances between the fiducial balls, then generates a best fitapproximation of the correlation between the sets of distances (step2224). If the distance correlation provides an approximation outside ofa preset tolerance (step 2226) then the program ends (steps 2218 and2220) having failed to automatically generate the transformationmatrices. If the correlation of distances is within tolerance and thereare seven groups (step 2228) then the image data is recorded in theimage matrix (step 2230). If the number of groups is above seven, then ageometry correlation is performed comparing the geometry of the groupsto the known geometry of the fiducial balls (step 2232). If the geometrycorrelation is successful (step 2234) then the transformation matricesare recorded (step 2230), and if not the program reports the errorcondition (step 2218).

Having successfully generated the image point matrix (step 2230), andsince the sensor point matrix is based on the known layout of thefiducial markers with respect to the transmitter, the mapping equationmay now be automatically generated as discussed above with reference toPowell's method.

In other embodiments wherein the patient is wearing a reference unitwhen the scan images are prerecorded the registration program mayautomatically locate portions of the reference unit itself on thescanned images, thereby identifying the orientation of the referenceunit with respect to the scanned images. Again, since the relativeorientation of the field generator with respect to the reference unit isknown, the registration unit may then generate the appropriate mappingfunction. In further embodiments the surfaces of the patient's skin maybe tracked such as by a laser light pointer or a movable tip pointerthat is biased in a forward direction. The tracked surfaces may then belocated on the stored images. In still further embodiments, theregistration unit could be programmed to identify characteristicstructures or features of the patient's body and thereby provide fullyautomatic registration. For example, the system might, knowing the sizeand shape of a headset, identify where the headset would be placed onthe patient's head, even though it does not appear on the prerecordedimages.

The position detection system may operate by any desired principlesuitable for generating a field in which position detection may beachieved at any location within the field. For example, it has beenfound that the 3 Space® Fastrak™ product sold by Polhemus, Incorporatedof Colchester, Vt. operates via principles suitable for use in thepresent invention. This product uses three orthogonally disposedmagnetic dipoles for both the transmitter and the sensor, and producesalternating electromagnetic fields of 8-14 kHz that are time divisionmultiplexed.

Specifically and with reference to FIG. 23, both the magnetic fieldsource 101 and the magnetic field sensor 103 include three orthogonallydisposed coils as shown. An alternating electric current from anamplifier 105 is passed through each of the source coils one at a timegenerating sequential magnetic fields. A processing unit 107 generatesthe timing signals and controls a digital-to-analog converter 109. Themagnetic fields induce voltages in the three coils of the sensor 103.The induced voltages are amplified by an amplifier 111, digitized by ananalog-to-digital converter 113, and then processed by the processingunit 107.

The time division multiplexed excitation of the three coils of thesource creates a unique magnetic field sequence throughout the field ofthe source. For every location in the field of the source, the sixdegree of freedom data can be calculated from the data present on thethree coils of the sensor. The six degree of freedom information is thensent to a host computer 115.

The position of a sensor S with respect to the field generator defininga reference coordinate frame (X,Y,Z) may be produced by the 3 Space®Fastrak™ product at a given time as a set of six values x_(s) y_(s),z_(s), ω_(azs), ω_(els), and ω_(ros). The values x_(s), y_(s), and z_(s)identify the position of the center of the sensor within the X,Y,Zcoordinate reference frame, and the angles ω_(azs), ω_(els), and ω_(ros)identify the orientation of the sensor S with respect to the X,Y,Zcoordinate reference frame.

The value ω_(azs) is the azimuth angle of the sensor. The azimuth angleidentifies the amount of rotation of the X and Y reference axes togetherabout the Z axis to a new position in which the X axis is aligned withthe center of the sensor in the Z direction. The new positions of the Xand Y axes are defined as X' and Y' respectively.

The value ω_(els) is the elevation angle of the sensor. The elevationangle identifies the amount of rotation of the X' and Z axes togetherabout the Y' axis to a new position in which the X' axis is aligned withthe center of the sensor S. The new positions of the X' and Z axes aredefined as X" and Z' respectively.

The value ω_(ros) is the roll angle of the sensor. The roll angleidentifies the amount of rotation of the Y' and Z' axes together aboutthe X" axis to a new position defining new axes Y" and Z" respectively.The sensor is oriented in the X",Y",Z" reference frame, and thisorientation is defined by the values ω_(azs), ω_(els), and ω_(ros).

The combined power of all the sensor data is inversely proportional tothe distance of the sensor from the source. The ratio between the sensordata components, created by the individual source coils, will determinethe x, y, z position coordinate of the sensor. The ratio between theindividual sensor coil data will determine the orientation of thesensor.

Because the medical instrument is free to move with respect to thetransmitter at speeds that may be faster than the rate at which theelectronics can process the information, the speed of the instrumentshould be monitored. If the speed of movement of the instrument is abovea defined threshold, then inconsistent sensor readings should be ignoreduntil the speed falls below the threshold. The speed may be monitored bycalculating a weighted sum of the differences between each of the x, y,and z coordinates at successive time intervals t₁ and t₂.

The presence of a signal from another source, or the magnetic field ofthe eddy current in a conductive object, or the field distorting effectof a ferro-magnetic object will change the magnitude / direction of theoriginal magnetic field of the source. This will result in an error inthe sensor position / orientation.

In a preferred embodiment involving field integrity detection and withreference to FIGS. 1-3, a reference sensor 37 may be securely mounted onthe transmitter assembly 12 at a fixed distance from the center of thetransmitter 36. The location and orientation of this reference sensorshould be determined through a calibration process under controlledconditions, and thereafter continuously calculated and verified. Incertain embodiments a weighted sum of all six sensor output parametersx_(s), y_(s), z_(s), ω_(azs), ω_(els), and ω_(ros) may be continuouslymonitored as an indication of compromised field integrity.

As also noted above and shown in FIGS. 7-12, the remote sensor 58 mayinclude a plurality of sensors (62,64) the outputs of which are comparedfor error detection purposes. Potential error conditions that would bedetectable by such a system include sensor failure where one sensorceases to operate properly, as well as uneven localized fielddistortions in the area of the medical instrument.

It has further been found that simply comparing the sensor outputs maynot sufficiently identify all types of error conditions that can occur,even if the distance between the sensors is taken into account. Such apotentially undetectable error condition may exist when a foreignferromagnetic object enters the electromagnetic field and producesidentical distortions at each of the sensors. This may be the case, forexample if the foreign object has uniform ferromagnetic properties, ifthe foreign object approaches the two sensors from the same distance andat the same rate, and if the sensors are equidistant from the generator.

In this situation the outputs of the sensors would produce identicaloutputs and an error detection signal might therefore not be producedeven though a foreign object would be in the electromagnetic fieldaltering the electromagnetic field as well as the sensed position data.Although the use of additional sensors may reduce the risk of thisoccurring, it does not eliminate the possibility of an error conditionbeing undetected.

It has been discovered that an error detection system sufficient toidentify localized uniform distortions in the area of the medicalinstrument or headset may be designed using two sensors separated by afixed distance as shown in FIGS. 7-12 and by monitoring the locations oftwo or more virtual points. As shown in FIG. 25, the sensors S₁ and S₂are separated from each other by a distance 2d and for conveniencedefined to be positioned along an axis such as the Y axis as shown.Sensor S₁ uniquely defines an X-Z plane in which it is located, and S₂uniquely defines an X-Z plane in which it is located as shown. A firstvirtual location v₁ is chosen to be between the X-Z planes defined bythe sensors, while a second virtual location v_(b) is chosen to beoutside of the X-Z planes defined by the sensors as shown in FIG. 11.The locations v_(a) and v_(b) are virtual locations that arecontinuously calculated and compared with factory defined positions.

In the embodiment diagrammatically shown in FIGS. 24 and 25 the virtualpoints v, (-d,-d,-d with respect to S₂) and v_(b) (d,d,d with respect toS₂) are equidistant from S₂. The sensor S₂ is the protected sensor inthis embodiment, and the sensor S₁ is used as a reference to provide theerror detection for S₂. The magnitude of the resultant vector from S₂ tov_(a) is the same as that from S₂ to v_(b) but opposite in direction,and this magnitude is approximately one half of the distance between S₁and S₂.

The locations of v_(a) and v_(b) in the reference coordinate system(i.e., with respect to S₁) must be calculated and will be referred to asv_(a1) and v_(b1). The location (PS) and the orientation of theprotected sensor (S1) with respect to the reference sensor must bedetermined. The attitude matrix (A) is calculated from the orientationvalues of the protected sensor: ##EQU5##

Then the locations of the virtual points are calculated as:

    v.sub.a1 =A·v.sub.a2 +PS

    v.sub.b1 =A·v.sub.b2 +PS

To establish a reference value for the virtual point location in thereference sensor coordinate system, a measurement is taken in adistortion free environment during factory calibration. These storedreference values are called v_(ae) and v_(be). Throughout the use of thesystem, the actual measured values of the virtual points (v_(am),v_(bm)) are compared to the stored reference values for the virtualpoints (v_(ae), v_(be)). If the distance between the established andmeasured location (Δ) for either virtual point is larger than a presetvalue (ε), then a field integrity violation message is displayed andnormal operation of the system is suspended. In particular and withreference to. FIG. 26

    |v.sub.alm -v.sub.ale |>ε or |v.sub.blm -v.sub.ble 51 >ε

The operation is based in part on the principle that if the positionerror is being reduced by the orientation error at one virtual point,then the error will be increased at the other Virtual point causing afield integrity violation signal to be generated. If for example, thereis an error in the measured position and orientation of the protectedsensor, then the measured value will have an error added to theestablished value. The field integrity checking is performed in thiscase as follows:

    |((A.sub.e +A.sub.mΔ)·v.sub.a2 +PS.sub.e +PS.sub.mΔ)-(A.sub.e ·v.sub.a2 +PS.sub.e)|>ε

or

    |((A.sub.e +A.sub.mΔ)·i v.sub.b2 +PS.sub.e +PS.sub.mΔ)-(A.sub.e ·v.sub.b2 +PS.sub.e)|>ε

which equals

    |A.sub.mΔ ·v.sub.a2 +PS.sub.mΔ |>ε

or

    |A.sub.mΔ ·v.sub.b2 +PS.sub.m66 |>.epsilon.

Substituting

    A.sub.mΔ ·v.sub.a2 =OPS.sub.amΔ and i A.sub.mΔ ·v.sub.b2 =OPS.sub.bmΔ

this relationship may be diagrammatically illustrated as shown in FIG.27. The tip location of the medical instrument should be initiallydefined with respect to the protected sensor (S2), and used indetermining the position of the tip with respect to the source. Theintegrity Of the field generated by the field generator may be monitoredas discussed above by positioning a reference sensor a fixed distancefrom the generator, and continuously monitoring its position for anychanges. The calculations involved in the above field integritydetection analysis regarding the two sensors S₁ and S₂, may be performedfor a transmitter and single sensor field integrity detection system.Specifically, the calculations may be performed by substituting thefield transmitter for the protected sensor (S₂), and by substituting thesingle sensor for the reference sensor (S₁). These field integrityanalyses may also be used to identify the half field of the operationenvironment.

As shown in FIG. 28 in alternative embodiments of the invention areference unit 120, including a field generator 122, may be positioned asmall distance away from the portion of the patient's body (such as thehead) 14 on an articulated arm 124. A headset 12 including a referencesensor 126 may be attached to the patient's body, and the medicalinstrument 16 may include a remote sensor 40 as discussed above withreference to FIGS. 1-6. Once the field generator 122 is positioned at aconvenient location it may be fixed in place by securing the joints ofthe articulated arm. The position of the patient with respect to thefield generator may accordingly be monitored. The position of theinstrument 16 with respect to the patient may also be determined and thesystem may then operate to display the appropriate prerecorded images asdiscussed below.

In various embodiments, the position of the field generator 88 may beadjusted during the surgical operation by moving the articulated joints.If neither the remote sensor 40 nor the reference sensor 126 are movedwith respect to one another, then moving the field generator 122 shouldnot affect the position detection system. If the accuracy of the systemdepends at all on the relative positions of the field generators 122 andthe sensors 40, 126, then it may be desirable to move the fieldgenerator 122 during the surgical operation. This may be the case, forexample, if the system relies on the near-field characteristics of amultiplexed magnetic field wherein it might be desirable to keep thesensors 40, 126 generally equidistant from the generator 122. In stillfurther embodiments, the system may periodically prompt the user toreposition the generator 122 such as through visual cues on the display.Those skilled in the art will appreciate that the relative positioningof the field generator and the one or more field sensors is in no waylimited to those shown.

The monitoring of the position of the patient may be accomplished bymeans other than using a headset and reference sensor. For example, acamera 128 connected to an image processor 130 may be positioned torecord the location of the field generator with respect to the targetoperation site of the patient as shown in FIG. 29. If either the patientor the field generator is moved, the image processor 130 will identifythe amount of relative change in location and advise the positiondetection unit 22 accordingly. Additional cameras positioned to view thepatient from a variety of directions may be employed in furtherembodiments.

As shown in FIG, 30 in an alternate embodiment, the system may include aflexible band 132 for secure attachment to a portion of a patient's body14 (e.g., a head or chest). The band 132 includes field generator 134and a reference sensor 136 that provides feedback to the signalgenerator in the position detection unit 22. The position detection unit22 is connected via communication lines 138 to the flexible band 132,and is connected via communication lines 140 to a flexible medicalinstrument 142 having a remote sensor at its tip 144. Because themedical instrument 142 is not rigid, the sensor should be positionedsufficiently close to the tip of the instrument 142 to provide accurateposition detection and monitoring within the patient's body. The display20 may indicate the relative orientation of the instrument 142 on one ormore images as shown.

As shown in FIGS. 31 and 32 a system of the invention may include aflexible medical instrument 150 having a sensor 152 at its distal tip154, and a fiber optic endoscope 156 having a sensor 158 at it distaltip 160. The fiber optic endoscope 156 is connected at its proximal endto a camera 162 which is in communication with an image processor 164.Because the field generator 134 on the reference band 132 may move, forexample as the patient breaths, the location of the remote sensor 152may appear to move when in fact the medical instrument 150 has notmoved.

To correct for this problem, the fiber optic endoscope 156 can be usedto monitor the position of the tip 154 of the instrument 150 withrespect to the inside of the patient's body as shown. Any sensedmovement of the sensor 152 with respect to the field generator 134 canbe evaluated with reference to whether the tip 154 has moved withrespect to the interior of the patient's body. If the camera observesthat the tip 154 has not moved, but the sensor 152 indicates that it hasmoved, then the system can identify that such movement was due to themovement of the field generator and not the sensor 152. The system maythen automatically correct for such variation. Further, the fiber opticendoscope 156 itself may include a sensor 158 for detecting whether thetip 160 of the fiber optic has moved. This should further enhance theaccuracy of the correction system. Also, the camera 162 may providecontinuous registration of the prerecorded images based on the internalstructure of the patient's body.

It will be understood by those skilled in the art that numerousvariations and modifications may be made to the above describedembodiments without departing from the spirit and scope of the presentinvention.

We claim:
 1. A system for monitoring the position of a medicalinstrument with respect to a patient's body, said system comprising:abody attachment unit securable to said patient's body; a medicalinstrument; a position characteristic field transmitter attached to oneof said body attachment unit and said medical instrument fortransmitting a position characteristic field; a position characteristicfield sensor unit attached to the other of said attachment unit and saidmedical instrument, said position characteristic field sensor unitincluding two sensors separated from one another by a fixed distance;and error detection means for identifying an error condition withrespect to said position characteristic field.
 2. A system as claimed inclaim 1, wherein said error detection means includes means fordetermining the positions of two virtual points, each of which lies in adifferent plane that is transverse to the axis joining the centers ofsaid sensors, wherein one of said planes is positioned between saidsensors and the other of said planes is not positioned between saidsensors.
 3. A system as claimed in claim 2, wherein said planes areequidistant from the center of one of said sensors.
 4. A system asclaimed in claim 2, wherein said virtual points are equidistant from thecenter of one of said sensors.
 5. A system as claimed in claim 2,wherein said means for determining the positions of two virtual pointsgenerates virtual point position information indicative of the positionsof said virtual points.
 6. A system as claimed in claim 5, wherein saiderror detection means further includes means for comparing said virtualpoint position information with known position data for said virtualpoints.
 7. A system for monitoring the position of a medical instrumentwith respect to a patient's body, said system comprising:a bodyattachment unit securable to said patient's body, said body attachmentunit including a position characteristic field transmitter attachedthereto for transmitting a position characteristic field; a medicalinstrument including first and second position characteristic fieldsensors separated from one another by a fixed distance; and errordetection means for identifying an error condition with respect to saidposition characteristic field, said error detection means includingmeans for determining the positions of two virtual points, each of whichlies in a different plane that is transverse to the axis joining thecenters of said sensors, wherein one of said planes is positionedbetween said sensors and the other of said planes is not positionedbetween said sensors.
 8. A system for monitoring the position of amedical instrument with respect to a patient's body and for displayingprerecorded computerized tomography scan images of said patient's bodyresponsive to the position of said medical instrument with respect tosaid patient's body, said system comprising:a body attachment unitsecurable to said patient's body, said body attachment unit including aposition characteristic field transmitter attached thereto fortransmitting a position characteristic field; a medical instrumentincluding first and second position characteristic field sensorsseparated from one another by a fixed distance; error detection meansfor identifying an error condition with respect to said positioncharacteristic field, said error detection means including means fordetermining the positions of two virtual points, each of which lies in adifferent plane that is transverse to the axis joining the centers ofsaid sensors, wherein one of said planes is positioned between saidsensors and the other of said planes is not positioned between saidsensors, and further including means for comparing measured positioninformation for said virtual points with prerecorded calibrationposition information for said virtual points; and display means fordisplaying prerecorded computerized tomography scan images.
 9. A systemas claimed in claim 8, wherein said display means is responsive to saiderror detection means for displaying an error condition identified bysaid error detection means.